Which is better to determine blockage or a previous heart attack, a thallium stress test or an angiogram?

What is the difference between a thallium stress test and an angiogram? Which one is best to determine blockage or a previous heart attack? Is the thallium test safe in regards to radiation levels or do both tests (thallium stress test and angiogram) pose radiation risks?

Coronary angiograms directly visualize the lumens (inner passageways) of the blood vessels that supply the heart with blood, i.e., the coronary arteries. Thallium studies (or technetium, another radioactive agent used in these tests) look to see if a radionuclide is taken up by heart muscle cells (normal) or if there is a portion of the heart that does not take it up. If there is a portion that does not take it up, this suggests either a prior heart attack (if at rest); or if during the stress portion of the test, this suggests compromise in the blood flow, i.e., a narrowing in one or more arteries. Either test is capable of diagnosing a previous heart attack. Both involve a small amount of radiation exposure.